Green pellet sizing screen

ABSTRACT

A green pellet sizing screen for classifying green or unfired taconite pellets according to size whereby undersize pellets fall through the screen and are returned for reprocessing while pellets of acceptable size are retained and deposited on a conveyor for delivery to the next processing station. The screen includes a plurality of parallel, spaced apart screen rods having an outer skin or shell of a material having properties of hard chromium and an inner core or body of a metal having a lower modulus of elasticity than the outer shell. The combination provides rods having a greatly extended service life over those of the prior art while providing better screening characteristics. The invention includes an alternative embodiment of a profile rod and a method of making the same.

[ Nov. 19, 1974 1 1 GREEN PELLET SIZING SCREEN [76] Inventor: James H. Flaherty, 3 SW. Third St.,

Grand Rapids, Minn. 55744 [22] Filed: Dec. 4, 1972 [21] App]. No.: 311,698

[52] US. Cl. 209/393, 209/406 [51] Int. Cl B071) H52 [58] Field of Search 209/392, 393, 254, 274, 209/406, 407, 408

[56] References Cited UNITED STATES PATENTS 703,683 7/1902 Vrooman 209/393 1,220,232 3/1917 Jackson 209/393 2,288,883 7/1942 Bixby 209/393 2,942,730 6/1960 Fontein.... 209/274 3,134,733 5/1964 Rose 209/393 3,154,622 10/1964 Reinfeld et a1. 209/393 X 3,255,884 6/1966 Sargent, Jr 209/254 FOREIGN PATENTS OR APPLICATIONS 928,504 6/1955 Germany 209/393 1,016,464 8/1952 France 209/392 K22,836 11/1955 Germany 209/393 292,054 6/1928 Great Britain 209/393 291,530 6/1928 Great Britain 209/393 OTHER PUBLICATIONS Article-How to Chromium-Plate for Greater Tool Life, By R. W. Bennet-Machinery Magazine of March, 1944pp. 190-194.

Primary Examiner-Harrison L. Hinson [5 7] ABSTRACT A green pellet sizing screen for classifying green or unfired taconite pellets according to size whereby undersize pellets fall through the screen and are returned for reprocessing while pellets of acceptable size are retained and deposited on a conveyor for delivery to 15 Claims, 12 Drawing Figures PATENTE HUVISIQH 3.848.744

SHEU 20? 2 FIG. 1

FIELJU GREEN PELLET SIZING SCREEN BACKGROUND OF THE INVENTION As reserves of high-grade iron ore are being depleted, the iron ore industry has turned to lower grade sources to obtain ore, one of them being taconite. In refining, taconite is crushed to a fine powder and the magnetic ore, or magnetite, is separated magnetically from the waste product, comprised mostly of silica. The magnetite is moistened to form a magnetite slurry concentrate. A clay product, as bentonite, may be added to the slurry to increase binding effectiveness. The concentrate is fed into a balling drum or disc which forms the concentrate into generally spherical green or unfired pellets. The balling drum comprises a large cylindrical drum having a generally horizontal but slightly inclined axis. As the drum rotates, particles of the concentrate form nuclei, and, due to the rotation and inclination of the drum, partake of a rolling action which results in the formation of balls which are commonly referred to as green pellets. The green pellets discharge through the lower end of the drum.

In order to facilitate further processing, it is desirable to maintain close sizing of the green pellets. To this end, upon exit from the balling drum the pellets are passed over a screen having a plurality of parallel screen elements or rods. The screen rods are spaced apart sufiiciently so that undersize pellets drop through onto a conveyor for reprocessing, while pellets of the proper size pass over the screen to a second conveyor for delivery to the next processing station.

Three embodiments of green pellet sizing screens find application in the industry. One comprises a vibrating screen deck as shown in US. Pat. No. 2,707,304 to Haley. A plurality of parallel spaced apart rods form a screen deck and suitable means are provided for vibration of the deck. As the pellets are discharged from the balling drum and dropped to the deck, the vibrating action of the deck induces the undersize pellets to fall through to a first conveyor for reprocessing while the proper size pellets roll off the end of the deck to a second conveyor for delivery to the next processing station.

A second type of sizing screen, known as a trommel screen, comprises a screen having a plurality of spaced apart profile rods shaped in rings coaxial to and rotatable with the drum. Such a screen is shown in US. Pat. No. 2,834,043 to Haley et al. The rotating action of the screen causes the undersize pellets to drop through while pellets of the proper size proceed to discharge through the end of the drum.

A third type, or roll screen, as shown in US. Pat. No. 2,988,781 to Meyer, is comprised of a deck having a plurality of cylindrical, spaced apart rods constituted as rollers, each being rotatable about its own longitudinal axis. Undersize pellets from the balling drum drop through the screen while proper size pellets are conveyed by the rotating rollers to the end of the screen as described in the aforementioned patent.

-Magnetite is an abrasive material having magnetic characteristics and likewise, the resultant green pellets are abrasive and magnetic. Screen rods of the prior art, for use in the aforementioned sizing screens, are typically fabricated of stainless steel having non-magnetic properties. Profile rods of ferromagnetic material are prone to collect bits of magnetite and other dirt which magnetically adhere to the rod, impeding the efficiency of the screening and accelerating wear of the rods through abrasion. Yet it would be desirable to fabricate the rods of material other than stainless steel. Stainless steel is expensive and difficult to weld reliably. The stainless steel rods wear through abrasion whereby green pellets of acceptable size are allowed to drop through the screen. Additionally, as wear takes place, strains on the outer skin fibers of the rods are relieved in such a manner that bowing of the rods occurs producing a varying spacing between the rods resulting in poor screening. When the useful life of the stainless steel rods of a screen is spent, the screen must be replaced, resulting not only in the expense of another screen, but expensive down time" at the mill.

SUMMARY OF THE INVENTION The present invention relates to pellet sizing screens having a useful service life far greater than those of the prior art. Pellet sizing screens according to the present invention include screen elements or rods having a hard pellet contacting portion and a protected inner core. In a preferred embodiment the rods have an outer skin or shell of a material having properties of hard chromium and an inner core or body of a material chosen for properties of fatigue resistance, cost, and workability. The hard outer shell attached to the inner core offers a high degree of resistance to abrasion and damage occasioned by the continuous flow of green pellets discharged from a balling drum and dropping on the screen. The screen rods are paramagnetic to inhibit the accumulation of magnetic concentrate particles and dirt on the screen. The outer shell remains highly polished to facilitate movement of pellets over the screen. The inner core is shock absorbent and fatigue resistant. The inner core is readily welded for fabrication and repair of the screens.

An object of the invention is to provide a sizing screen having a greatly extended service life over those of the prior art. A second object is to provide such a screen which will maintain a uniform and constant spacing between screen rods for uniform screening results. A further object is to provide such a screen having shock absorbent and fatigue resistant characteristics. A yet further object is to provide such a screen having rods easily worked and welded to faciliatefabrication and repair of the screen. A yet further object of the invention is to provide a method for the construction of an inexpensive profile rod. Other objects of the invention will become apparent upon the following description.

IN THE DRAWINGS FIG. 1 is a top plan view of a balling drum and a first embodiment of a pellet sizing screen of the invention;

FIG. 2 is a cross-sectional view of several of the screen rods of the sizing screen of FIG. 1;

FIG. 3 is an enlarged fragmentary view of one of the screen rods of FIG. 2;

FIG. 4 is a side elevational view of a balling drum having a pellet sizing screen according to a second embodiment of the invention;

FIG. 5 is a cross-sectional view of several of the profile rods of the sizing screen of FIG. 4;

FIG. 6 is a top plan view of a pellet sizing screen according to a third embodiment of the invention;

FIG. 7 is a cross-sectional view of several of the screen rods of the sizing screen of FIG. 6;

FIG. 8 is a fragmentary perspective view of bars in preparation for forming into profile rods according to a method of the invention;

FIG. 9 is a fragmentary perspective view of profile rods formed according to a method of the invention;

FIG. 10 is a cross-sectional view illustrating formation of profile rods according to the present invention;

FIG. 11 is a cross-sectional view of a profile rod of an alternative embodiment formed according to the method of the invention; and

FIG. 12 is a cross-sectional view of a further alternative embodiment of a profile rod of the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS Referring to the drawings, there is shown in FIG. 1 a cylindrical rotary pellet forming drum or balling drum 10 and a pellet sizing screen deck 11 located in receivng relationship to the discharge end 13 of the drum 10. Drum 10, of the type described in US. Pat. No. 2,707,304 to Haley, is open at both ends, having a generally horizontal axis but inclined slightly downward to promote movement of the forming pellets toward the discharge end. Suitable conventional means are provided for rotation of the drum about its axis, as by providing tracks 14 which engage suitable rollers (not shown) and a suitable motor and gear assembly to drive a ring gear 15 for rotation of the drum as indicated by the arrow 17.

Screen 11 includes a plurality of parallel, elongated, spaced apart members or screen rods 18 secured in relative arrangement by appropriate frame members 19 forming the screen deck 1 1. The flat upper surface defined by the deck is inclined relative to the horizontal toward a conveyor belt 21 as will be more fully explained. Means are provided to impart movement or vibration to the screen deck 11 and to this end a motor 22 drives a pulley and belt engaging an eccentric pulley 23 rotatably connected to suitable frame members (not shown) which hold and support screen deck 11 adjacent the discharge end 13 of drum 10.

Magnetite concentrate is introduced to the drum 10 through the upper end opposite discharge end 13. As the drum rotates, small nuclei of concentrate are formed which, proceeding down the length of the drum, grow progressively larger into roughly spherical shaped pellets. The green pellets exit the discharge end 13 of the drum, falling upon the screen deck 11. As shown by FIG. 2, pellets 27 of an acceptable size are retained by the screen deck 11 while undersize pellets 28 fall through the screen. The spacing between the profile rods is determinative of the size of pellet retained and should be kept as uniform and constant as possible.

The undersize pellets which fall through the screen deck 11 are deposited on a laterally disposed conveyor 25 running beneath the screen. Conveyor 25 delivers the pellets to a traveling belt 26 which returns the undersize material for reprocessing. Acceptable pellets 27 travel along the screen deck 11 and land on conveyor 21 for delivery to the next processing station, usually a kiln. The vibrating action of the screen deck induces the lateral movement of the pellets and prevents clogging of the screen.

The screen rods 18 of the screen 11 of the invention account for the dramatically increased life span of the screen over those of the prior art. As shown by FIG. 3, each rod includes an inner core or body 31 and an outer shell or skin 32. The inner core 31 is a material, such as from plain carbon steel or nickel or copper alloys, chosen for properties of fatigue resistance, lowcost, and workability. Inner core 31 may be ferromagnetic, thereby allowing a wide choice of suitable metals and alloys. Outer shell 32 attached to inner core 31 is a material having properties of, and in the preferred embodiment consisting of, hard chromium. Together the inner core 31 and outer shell 32 produce a screen rod of far greater durability and less expense than those of the prior art. The outer shell 32, being of hard chromium, is extremely hard and has a low surface energy, thus resisting the abrading wear of the green pellets. Taconite concentrate is acidic which would normally accelerate corrosion of the rod. However, the outer shell of hard chromium resists corrosion well, and is attacked by few chemicals or substances, thus protecting the inner core and prolonging the rod life. The outer shell is paramagnetic to shield the ferromagnetic inner core and prevent the accumulation of magnetic concentrate particles on the screen. The outer shell of hard chromium is smooth and may be highly polished whereby the green pellets move easily along the screen. Due to the low surface energy, the outer shell 32 is, to an extent, self-polishing as material moves over it. The inner core 31 will generally be extruded whereby extruding grooves, as 34, may result, which are covered or reduced by the outer shell 32. The inner core is readily welded for ease in fabrication and repair of screens. The stiffness produced by the combination of the hard outer shell 32 with its high modulus of elasticity and the inner core has an effect on the natural frequency of vibration of the profile rod. Amplitudes of vibration and excursion of the screen rods are reduced because of the stiffer spring rate. This stiffness reduces the strain on the welds and thereby increases fatigue life. The stiffness also maintains the rods to a high degree of parallelism and thereby controls the design spacing to a much better degree. A better control on' pellet sizing is thus achieved.

On screen rods of the prior art, as wear takes place, strains on the outer skin fiber are relieved in such a manner that bowing of the rod occurs producing a varying space between rods resulting in poor screening. As wear takes place on the outer shell 32 of the screen rod of the present invention, only minimal fiber skin stresses occur and these are not transmitted to the inner core 31, preventing such bowing. As the outer shell 32 wears, eventually space between rods may begin to widen and the paramagnetic shield provided by the outer shell decreases in effectiveness. The rod is simply removed and the outer shell renewed as by plating, then replaced.

In the use of pellet sizing screens, it is common to use mechanical hammers or wrappers which automatically strike the screen deck at preselected intervals of time to jar loose pellets which may lodge in the screen or other particles that may accumulate on the screen. Such blows cause fatigue of the screen rods in addition to that occasioned by the continual flow of pellets. Screen rods of the present invention offer a high degree of shock resistance. Shocks imparted to the rods are absorbed by the inner core 31 which is a material softer than the outer shell 32. The outer shell, at the same time, physically shields the inner core impeding wear and damage. The fatigue characteristics of the rod are greatly enhanced.

In the preferred embodiment, the outer shell is applied to the inner core by standard electrolytic processes. However, the outer shell could be applied by other suitable methods as fusion at high temperatures, sintering, or application through a plasma arc. An outer shell of hard chromium is preferable although other materials having desirable properties approximating those of hard chromium are usable. For example, these may include tungsten carbide, Pyrolite carbon, or molybdenum, all having a modulus of elasticity in excess of 40 million pounds per square inch and a hardness equal to or exceeding 70 Rockwell C.

The inner core 31 will generally be a metal chosen for the aforementioned properties including fatigue resistance, lightweight, workability and cost. Certain non-metallic materials could be employed and, in the rapidly advancing art of materials, it is foreseeable that a processed or synthetic non-metallic material could be developed possessing suitable properties for use as the inner core 31. In certain applications where a screen deck is to be heated, as for drying pellets, or cooled, the material for the inner core will be chosen for high thermal conductivity as well as other properties.

FIG. 4 shows a side elevational view of a balling drum and pellet sizing screen according to a second embodiment of the invention. A balling drum 37, of the type described in US Pat. No. 2,834,043 to Haley et al., has a pair of tracks 38 and a ring gear 39 for rotation by suitable conventional means (not shown) about its central axis. The central axis of balling drum 37 is generally horizontal but is actually inclined slightly from left to right when viewed as in FIG. 4, whereby gravity moves the green pellets along from the inlet to the discharge end of the drum as previously described.

The discharge end of the drum 37 extends into a green pellet sizing screen 41 of the invention rotatable with the drum. Screen 41 is comprised of a plurality of spaced profile rods 42 each in the form of a closed ring coaxial with the drum and with the plane thereof perpendicular to the axis of the drum. The ring adjacent the end of the drum 37 is substantially the same diameter thereof, successive rings progressively increasing in diameter, as shown. A cylindrical section 43 extends outward from the outermost ring. Suitable frame members 45 secure the rings to one another as well as to the drum 37 and cylindrical section 43.

As previously described, magnetite concentrate is introduced into the inlet end of the drum. Green pellets are formed by the rolling action of the drum. The pellets pass through the discharge end of the drum over the screen 41. As shown in FIGS. 4 and 5, undersize pellets 46 fall through the screen while pellets 47 of an acceptable size are retained by the screen. Undersize pellets 46 fall to a conveyor belt 49 for return and reprocessing. Acceptable pellets 47 are discharged through cylindrical section 43 to a traveling belt 50 for delivery to the next processing station.

Referring to FIG. 5, each of the profile rods 42 is gen.- erally T-shaped and has an outer shell or skin 52 and an inner core or body 53. The T-shape of the profile rod 42 furnishes a greater expanse of opposing surfaces in the space between the rods whereby uniform spacing between adjacent rods is maintained longer. Outer shell 52 is hard chromium while inner core 53 is a metal chosen for the properties of fatigue resistance, workability and the like, as previously described, not being limited by a paramagnetic criteria. The aforementioned advantages are again realized. The life of the screen 41 is greatly lengthened over those of the prior art. A high degree of parallelism and uniform spacing of the rods is maintained. The rods are easily welded for ease of fabrication and repair. Buildup of dirt and magnetic particles is avoided. The profile rods resist corrosion and abrasion. As the outer shell 52 begins to wear, it is readily renewed. Time and money are both saved.

FIG. 6 illustrates a top plan view of a screen deck comprising a third embodiment of the invention. A screen deck 55, of the type shown in US. Pat. No. 2,988,781 to Meyer, includes a plurality of parallel, spaced apart rods constituted as cylindrical rollers 56. The rollers 56 are joumaled at either end and rotatably assembled in and supported by a frame 57. Each of the rollers 56 is individually rotatable in the frame 57 and is provided with a sprocket wheel 59 for engaging a driving chain (not shown). Because of the close spacing of the rollers, alternate sprocket wheels are staggered. The drive chain is adapted to simultaneously drive all of the rollers in the same direction of rotation.

Green pellets are deposited on the screen deck 55 from a balling machine (not shown) of the type shown in FIG. 1. As shown by FIG. 7, undersize pellets 60 fall through the screen deck 55 to an appropriate conveyor for return and reprocessing. Pellets 61 of an acceptable size are conveyed by the rotating action of the rollers 56, as described in the aforementioned patent, to a traveling belt for delivery to the next processing station.

Each of the rollers 56 is comprised of an outer shell or skin 63 and an inner core or body 64. The outer shell 63 has properties of hard chromium and the inner shell 64 is a metal chosen for the aforementioned desirable characteristics. The same extended life span of the screen deck 55 as well as the other desirable characteristics are again achieved. The outer surface of the rollers 56 of the screen deck 55 may be roughened, if desired, to improve traction and expedite movement of green pellets over the screen.

In the use of a screen deck as shown in FIG. 6, typically the green pellets are discharged onto the central I portion of the screen. The action of the pellets rolling over the sceeen keeps the screen free of accumulated particles of concentrate and dirt and the like. However, a residue may build up on the less used end portions of the rods to an extent whereby adjacent rotating rods come in contact with one another. If this occurs, one roller may sear another seriously damaging it and greatly decreasing the life span. The hard outer shell of the rollers of the present invention provide protection against such damage and, if desired, the width of the outer shell 63 may be increased near the ends of the rollers for added protection.

In practice, pellet sizing screens of the types described having stainless steel profile rods have a useful service life of 1,200 to 1,700 hours. At the end of the useful service life, the balling mill must be shut down while the screen is replaced. Such down time is a significant expense in addition to the expense of a new screen. In practice, screens constructed accordding to the present invention have a useful service life in excess of 4,400 hours with substantial savings of time and money. Toward the end of the useful life of a screen, the outer shells of the profile rods may be renewed to further extend the life of the screen.

It thus may be seen that the provision of a profile rod according to the present invention having an outer shell with the properties of hard chromium and a metallic inner core produces pellet sizing screens far superior to those of the prior art. Normally the metal of the inner core will be chosen for properties of fatigue resistance, workability, as good welding characteristics, and a modulus of elasticity, for example, of approximately 28 million pounds per square inch. As the ferromagnetic characteristics of the metal chosen are of no concern, a wider range of choices is available. However, nonmagnetic metals such as aluminum, stainless steel, and copper alloys are also usable as the inner core. As shown in the drawings, for purposes of illustration the width of the outer shell is exaggerated. In practice, an outer shell having a width of 0.005 inch to 0.010 inch has proven satisfactory.

As shown in FIG. 2, the screen rods of the invention have a circular cross section while the profile rods of the embodiment shown in FIG. have a generally T- shaped cross section. The use of such profile rods is preferable over the use of circular rods. The spacing between adjacent circular rods is the distance between a single point on the circumference of one rod to a point on the circumference of the adjacent rod. As the rod wears, the spacing between the respective points increases, reducing screening efficiency. The spacing between adjacent profile rods having a T-shaped cross section is determined by the distance between opposing surfaces. The spacing remains constant until the entire surface is worn, providing an increased screen life. The vertical dimension of the opposing surfaces should not be so large as to cause frequent clogging of the screen. The lower narrow portion of the cross section of the profile rod serves to increase the cross-sectional area of the rod, thereby increasing the moment of inertia of the cross section. Greater strength in bending and fatigue resistance is achieved.

Profile rods must be specially extruded or machined, whereas circular rods are readily available. Therefore, despite the advantages achieved through the use of profile rods, circular screen rods are in common use due to the greatly increased cost of profile rods over screen rods. Accordingly, an alternative embodiment of the invention, as shown in FIGS. 8 through 10, comprises an inexpensive profile rod and a method of making the same.

Referring to FIG. 8, there is shown a cross-sectional fragmentary view of a plurality of rods 68, each having an upper cap or outer shell 69 and a rectangular body or inner core 70. The rods 68 are assembled in a screen deck (not shown) as, for example, the deck shown in FIG. 1. The rods are relatively spaced to allow passage of undersize pellets while retaining pellets of the proper size. The outer shell 69 is of a material having the properties of hard chromium. Rectangular core 70 is of a material chosen for properties of fatigue resistance, workability, and the like, as previously described. Core 70 may be, for example, a relatively inexpensive and readily available carbon steel rod. The outer shell 69 is attached to the upper portion of the core 70 covering the top surface and upper extremities of the sides, extending for a selected distance down either side of the core. Outer shell 69 is applied by any usual or preferred means, as by masking that portion of the bar not to be covered and subjecting the remainder of the bar to an electrolytic process.

In terms of a method of making a profile rod, green pellets are discharged from a balling drum and deposited on the screen of the rods of FIG. 8. As shown by FIG. 10, undersize pellets 72 fall through the screen.

while pellets 73 of a proper size are retained by the screen and delivered to an appropriate conveying means for transfer to the next processing station. As the abrasive pellets fall through the screen, the lower portion 75 of the rods, unprotected by the outer shell 69, erode and, after a few hours of screeening, assume a profile as that shown in FIG. 9 and in phantom in FIG. 10.

The resultant profile rods, as shown in FIG. 9, are generally of the T-shape type having a head 78 covered and protected by the outer shell 69, and a narrowed portion or neck 79 having a reduced transverse dimension. The aforementioned advantages of profile rods arerealized. The opposing vertical surfaces of the rods provide uniform screening and an increased screen life. The additional cross-sectional area provided by the neck 79 increases the bending strength and fatigue resistance of the rods.

It is understood that during the initial forming of the profile rods, from a configuration as shown in FIG. 8 to a configuration as shown in FIG. 9, the screen will run dirty, that is, the screen will frequently clog with pel lets and material lodged between the opposing vertical surfaces of the rods. Initially, the screen will require frequent cleaning. After a few hours of screening, however, the rods forrn the profile shown in FIG. 9 and the clogging of the screen is no longer a significant problem.

An additional embodiment of a profile rod according to the present invention is shown in cross section in FIG. 11. The profile rod of FIG. 11 is constituted from a linear, rectangular metal bar 81, as steel, having the upper portion 82 heat treated to a higher degree than the lower portion 84. This may be accomplished, for example, during the quenching process. The upper portion is thus significantly harder than the lower portion. The profile rod 81 when disposed in a screen of such rods, wears in a similar fashion to the profile rods shown in FIGS. 8 through 10. The lower peripheral portion of the rod, as indicated at 85, is worn away through abrasion by the green pellets, and the generally T-shaped profile is assumed, including a head 87 and a lower necked-in body portion indicated in phantom at 88. The hard head resists wear through abrasion by the screen pellets, while the increased cross-sectional area provided by the necked-in portion increases the bending strength and fatigue resistance of the rod. While the rod 81 will not be as durable as those having an outer shell of hard chromium, it is apparent that there is provided a profile rod of equal durability to those of the prior art but far more economical to fabricate and use.

It is apparent that with a sizing screen having profile rods of the type shown in FIG. 9, when wear of the outer shell 69 takes place, the point of first and greatest wear will be the comers. When a corner is finally worn through, rapid deterioration of the inner core will occur. Accordingly, in FIG. 12 there is shown in cross section an additional embodiment of a profile rod according to the invention which alleviates this problem and extends the wear life of the profile rod an even greater degree. There is provided a profile rod 90 having an inner core 91 and an outer shell 92 adhering to and covering the top and upper extremities of the sides of inner core 91. The outer shell 92 defines a rectangular profile with a horizontal top and vertical sides. The top of the inner core 91 is rounded, preferably circular, whereby an additional amount of material of the outer shell 92 is located proximate the corners, as at 94. As a corner begins to wear, as indicated in phantom at 95, a greater wear surface of the hard outer shell 92 is exposed resulting in greater wear life of the profile rod. The lower neck portion 96 of the inner core may, if desired, be fabricated according to the method of fabrication described relative to the profile rods of FIG. 9.

I claim:

1. A pellet sizing screen of the type disposed adjacent the discharge end of a balling drum to classify magnetite type iron oxide pellets formed in the balling drum of magnetite concentrate according to size and usable in conjunction with means to impart movement to the screen, said screen comprising:

a plurality of screen rods relatively spaced apart to define openings of sufficient size to allow passage of undersize magnetite type pellets and restrict passage of magnetite type pellets equal to or greater than a preselected size;

means securing the screen rods in relative arrangement to one another;

each said screen rod having an inner core and an outer shell attached to at least a portion of the inner core;

said outer shell being of hard chromium of a thickness of at least about 0.005 inch and adapted to receive and convey magnetite type pellets discharged from the balling drum; and

said inner core being a fatigue resistant material hav ing a modulus of elasticity lower than the material of the outer shell.

2. The screen of claim 1 wherein: on each said screen rod the outer shell surrounds the inner core.

3. The screen of claim 2 wherein: said screen rods are linear and parallel defining a flat deck locatable adjacent the discharge end of a balling drum at an inclination; said deck adapted to be vibrated by vibrating means.

4. The screen of claim 2 wherein: said screen rods form a plurality of spaced apart, closed rings coaxial to and rotatable with said drum disposed adjacent the discharge end of said drum.

5. The screen of claim 2 wherein: said screen rods are linear and parallel and mounted in a frame to define a horizontal screen deck, each rod being individually rotatable about its longitudinal axis relative to said frame, said rods adapted to be simultaneously rotated in the same direction.

6. The screen of claim 2 wherein: said inner core is a material having a modulus of elasticity of approximately 28 million pounds per square inch.

7. The screen of claim 2 wherein: said inner core is a ferromagnetic metal.

8. The screen of claim 1 wherein: the inner core of each said screen rod has an upper head portion having vertical sides, and a lower neck portion, said neck portion being narrower than said head portion, said outer shell attached to and covering said head portion.

9. The pellet sizing screen of claim 1 wherein: said inner core has an upper head portion having vertical sides, and a lower neck portion, said neck portion being narrower than the head portion, said outer shell attached to and covering said head portion.

10. The pellet sizing screen of claim 1 wherein: said inner core has a rounded upper head portion and a lower neck portion narrower than the head portion, said outer shell attached to and covering the upper head portion.

11. The pellet sizing screen claim 13 wherein: said outer shell has a horizontal top surface and vertical side surfaces.

12. An apparatus for forming magnetite type iron oxide pellets and classifying them according to size, comprising:

a balling drum having an open inlet end for the introduction of magnetite type concentrate therein and a discharge end for discharge of magnetite particles;

means for the rotation of said balling drum to cause formation of magnetite type pellets;

a pellet sizing screen located adjacent the discharge end of said balling drum to classify the magnetite type pellets formed in the balling drum according to size;

said sizing screen including a plurality of screen rods relatively spaced apart to define openings of sufficient size to allow passage of undersize pellets and restrict passage of pellets equal to or greater than a preselected size;

means securing the screen rods in relative arrangement to one another;

each said screen rod having an inner core and an outer shell attached to at least a portion of the inner core;

said outer shell being of hard chromium of a thickness of at least about 0.005 inch and adapted to receive and convey magnetite type pellets discharged from the balling drum; and

said inner core being a fatigue resistant material having a modulus of elasticity lower than the material of the outer shell.

13. The apparatus of claim 12 wherein: said screen rods are linear and parallel defining a flat deck locatable adjacent the discharge end of said balling drum at an inclination; said deck adapted to be vibrated by vibrating means.

14. The apparatus of claim 12 wherein: said screen rods form a plurality of spaced apart, closed rings coaxial to and rotatable with said drum disposed adjacent the discharge end of said drum.

15. The apparatus of claim 12 wherein: said screen rods are linear and parallel and mounted in a frame to define a horizontal screen deck, each rod being individually rotatable about its longitudinal axis relative to said frame, said rods adapted to be simultaneously rotated in the same direction.

P0405) UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,848,744 Dated November 19, 1974 Inventor) James H. Flaherty It is certified that error appears in the'above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 3, line 21, "receivng" should be --receiving-.

Column 6, line 65, "accordding" should be "according".

Claim 11, line 1 ,-after "screen" -of-dis omitted;

vsam'e'd'line, "13" should be -l0.

Signed and sealed this 21st day of January 1975.

(SEAL) Attest:

MCCOY GIBSON JR. 4 C. MARSHALL DANN Attestlng Officer Commissioner of Patents 

1. A pellet sizing screen of the type disposed adjacent the discharge end of a balling drum to classify magnetite type iron oxide pellets formed in the balling drum of magnetite concentrate according to size and usable in conjunction with means to impart movement to the screen, said screen comprising: a plurality of screen rods relatively spaced apart to define openings of sufficient size to allow passage of undersize magnetite type pellets and restrict passage of magnetite type pellets equal to or greater than a preselected size; means securing the screen rods in relative arrangement to one another; each said screen rod having an inner core and an outer shell attached to at least a portion of the inner core; said outer shell being of hard chromium of a thickness of at least about 0.005 inch and adapted to receive and convey magnetite type pellets discharged from the balling drum; and said inner core being a fatigue resistant material having a modulus of elasticity lower than the material of the outer shell.
 2. The screen of claim 1 wherein: on each said screen rod the outer shell surrounds the inner core.
 3. The screen of claim 2 wherein: said screen rods are linear and parallel defining a flat deck locatable adjacent the discharge end of a balling drum at an inclination; said deck adapted to be vibrated by vibrating means.
 4. The screen of claim 2 wherein: said screen rods form a plurality of spaced apart, closed rings coaxial to and rotatable with said drum disposed adjacent the discharge end of said drum.
 5. The screen of claim 2 wherein: said screen rods are linear and parallel and mounted in a frame to define a horizontal screen deck, each rod being individually rotatable about its longitudinal axis relative to said frame, said rods adapted to be simultaneously rotated in the same direction.
 6. The screen of claim 2 wherein: said inner core is a material having a modulus of elasticity of approximately 28 million pounds per square inch.
 7. The screen of claim 2 wherein: said inner core is a ferromagnetic metal.
 8. The screen of claim 1 wherein: the inner core of each said screen rod has an upper head portion having vertical sides, and a lower neck portion, said neck portion being narrower than said head portion, said outer shell attached to and covering said head portion.
 9. The pellet sizing screen of claim 1 wherein: said inner core has an upper head portion having vertical sides, and a lower neck portion, said neck portion being narrower than the head portion, said outer shell attached to and covering said head portion.
 10. The pellet sizing screen of claim 1 wherein: said inner core has a rounded upper head portion and a lower neck portion narrower than the head portion, said outer shell attached to and covering the upper head portion.
 11. The pellet sizing screen claim 13 wherein: said outer shell has a horizontal top surface and vertical side surfaces.
 12. An apparatus for forming magnetite type iron oxide pellets and classifying them according to size, comprising: a balling drum having an open inlet end for the introduction of magnetite type concentrate therein and a discharge end for discharge of magnetite particles; means for the rotation of said balling drum to cause formation of magnetite type pellets; a pellet sizing screen located adjacent the discharge end of said balling drum to classify the magnetite type pellets formed in the balling drum according to size; said sizing screen including a plurality of screen rods relatively spaced apart to define openings of sufficient size to allow passage of undersize pellets and restrict passage of pellets equal to or greater than a preselected size; means securing the screen rods in relative arrangement to one another; each said screen rod having an inner core and an outer shell attached to at least a portion of the inner core; said outer shell being of hard chromium of a thickness of at least about 0.005 inch and adapted to receive and convey magnetite type pellets discharged from the balling drum; and said inner core being a fatigue resistant material having a modulus of elasticity lower than the material of the outer shell.
 13. The apparatus of claim 12 wherein: said screen rods are linear and parallel defining a flat deck locatable adjacent the discharge end of said balling drum at an inclination; said deck adapted to be vibrated by vibrating means.
 14. The apparatus of claim 12 wherein: said screen rods form a plurality of spaced apart, closed rings coaxial to and rotatable with said drum disposed adjacent the discharge end of said drum.
 15. The apparatus of claim 12 wherein: said screen rods are linear and parallel and mounted in a frame to define a horizontal screen deck, each rod being individually rotatable about its longitudinal axis relative to said frame, said rods adapted to be simultaneously rotated in the same direction. 